Outcrop Analogues for Lower Paleozoic Hydrothermal Dolomite Reservoirs, Mohawk Valley, New York

Smith, Langhorne and Richard Nyahay
Reservoir Characterization Group, New York State Museum, Albany, NY

Field relations, petrography and geochemical analysis of Lower Paleozoic carbonate outcrops in the Mohawk Valley of New York suggest that they have undergone significant fault-related hydrothermal diagenesis. The outcrops are in the Cambrian Little Falls and Lower Ordovician Tribes Hill Formations, which are both part of the Beekmantown Group. The Beekmantown is the equivalent of the Knox, Arbuckle and Ellenburger Groups, which have all produced oil and gas to the south and west of New York.

Fault-related hydrothermal alteration occurs when fluids flow up active faults and into porous and permeable strata that underlie seals near the surface. The Beekmantown Group underlies the Trenton-Black River carbonates, which host prolific hydrothermal dolomite reservoirs in New York. Because the fluids flow from the bottom up, the Beekmantown has experienced similar, if not more intense, hydrothermal alteration and is therefore a valid analogue for the Trenton Black River and is a legitimate exploration target in itself.

Evidence for fault-related hydrothermal alteration includes: fracture-, vug- and pore-filling saddle dolomite, chalcedony, anthraxolite, calcite, sphalerite and pyrite, matrix dolomitization around faults and fractures and extensive brecciation and fracturing. Breccias formed by faulting and thermobaric fluid flow are differentiated from meteoric karst breccias by the composition of their matrix and their stratigraphic and structural position. Fault-related hydrothermal dolomite is differentiated from early “top-down” dolomite by its geochemistry and field relations.

Alteration occurs in multiple structural settings related to the Late Ordovician Taconic Orogeny. Dolomitization and brecciation occur around en echelon faults and anticlines related to strike slip fault movement. There is also extensive alteration around normal faults that formed or were reactivated during Taconic flexural extension in the foreland basin. The structural settings and fault-related diagenesis are analogous to Trenton-Black River reservoirs and hydrothermally altered reservoirs around the world.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004